No BuildMachines

This branch gets rid of all in-hydra management of build machines. Keeping two separate databases of build machine information safely in sync is more work than it's worth, especially since the build hook doesn't care about the BuildMachines. Instead, when build machine information is needed, parse it out of /etc/nix.machines (checking NIX_REMOTE_SYSTEMS first). Administrators should manage /etc/nix.machines out-of-band.

Along similar lines, I plan on getting rid of the SystemTypes table and having the queue runner use nix.machines to see the number of build hooks per system.